I have tried quite a few of the various maps available for use on my Garmin device, as of yet I have been unable to find one that looks exactly the same as the basemap!
is there one available for the uk?

Garmin devices let you set the amount of details to be shown and also the size of street and place names etc.

Also a map displayed in a browser window can show a larger amount of details and POIs without confusing the user. A GPS receiver would likely slow down when rendering all of this information.

In contrast maps built for Garmin devices are usually specialised on a certain task, e.g. a bicycle map might have a layer for contour lines, official cycle routes and also shows repair stations and other relevant POIs.

So just look for a build that covers your needs or build a Garmin map yourself.